B.2 Test de Morisky-Green-Levine…

Figure 5.4: (a) Cationic chains of [Fe(Htrz)2(trz)](ClO4) with Fe2+aligned parallel along the b axis with the three bridging triazoles in alternating invert positions (ClO−₄ anions are omitted for clarity). (b) BF−₄ anion cavities in the crystal packing of [Fe(Htrz)2(trz)](ClO4). Figure courtesy of Mónica Giménez.

Polymeric chains of Fe2+ ions situated along the b axis with the three bridg- ing triazoles in alternating invert positions are the base of the crystal structure with two Htrz ligands and one trz− ligand (Fig. 5.4 a)), giving the formula [Fe2+(Htrz)₂(trz−)](ClO−₄) (Pnma space group). It can be found each ClO−₄ ion located in cavities formed by the triazole ligands (Fig. 5.4 b)) and no water molecules are found within the crystal structure. The distance between two next iron centres within a chain is half of the b parameter. Each [Fe(Htrz)₂(trz)]n

chain is surrounded by six identical chains with two different Fe· · · Fe interchain distances [2]. This structural study confirms a chain architecture formed by [Fe(Htrz)₂(trz)]n chains and connected through short N-H· · · N contacts. There-

fore, the unusually abrupt transition and large hysteresis of 40 K above room temperatures is consequently ascribed to this structural characterization. De- scriptions and magnetic measurement of this kind of NPs can be found in [11]. Studies carried out by Coronado et al. showed that the magnetic hysteresis of Sample A NPs can be tuned with modifications of the chemical composition [12,13] based on the concept of molecular alloy proposed by Kahn which drives to a fam- ily of coordination polymers of general formula [Fe(Htrz)_3−3x(NH₂trz)_3x](BF₄)₂. The resulting compound will show a variable transition temperature in the bulk, depending on the composition [14].

Using this strategy NPS with bi-stability closer to room temperature can be ob- tained. For example, the ligand can be replaced to promote minor changes in the chemical composition (0.016 < x < 0.1). The thermal hysteresis from a series of samples is demonstrated to move to lower temperatures as the ratio of amino- triazole increases.

Raman spectroscopy is of great interest since it allows to reproduce the hystere- sis loops (χMT vs T curve) saving a great amount of time. Other advantage

Chapter 5. Raman spectroscopy for the study of Spin-Crossover in iron Fe2+/Fe3+complexes

of the Raman spectrocopy is that the crossover can be monitored on really small samples, less than 1 microgram. While magnetic measurements would require few milligrams to obtain trustable data. This will be demonstrated in the next section during the study of microcrystals. Importantly, our results suggest the possibility of characterization of spin states in isolated NPs, as the first step towards the optical readout of single magnet memories.

5.2.2 Results and discussion

Magnetic and optical characterization of the NP under investigation can be found in [2]. Both kinds of NPs present similar behavior with a narrow transition from LS to HS as the temperature is increased, see table 5.1. This data are extracted from the thermal dependence of χMT for both thermal spin transitions

are indicated as, T↑_1/2 and T↓_1/2 while ΔT represents the hysteresis width that appears in all the spin transitions. It is worth to mention that, for the NPs studied at our group, the crossover properties (e.g. ΔT ) does not differ to much from the bulk values. Obviously, the hysteresis and transition temperature can be slightly tuned by changing the NP size but the main control is done through the ligand. For this reason, in this section we did not pay attention to the NP size. Just to complete information we have got average sizes of 4 and 10 nm for Samples A and B respectively.

Bulk A Sample A Bulk B Sample B T↑_1/2 391 373 313 315 T↓_1/2 349 343 296 305 ΔT 42 30 17 10

Table 5.1: Physical characteristics of the thermal spin transition T↑_1/2and T↓_1/2for both samples and bulk. Data courtesy of Mónica Giménez

NPs of Sample A exhibit a reduced hysteresis loop of 30 K (T↑_1/2 = 373 K and T↓_1/2 = 343 K). Typically for these nanostructured chain compounds it can be ascribed to the different coordination environment of the terminal Fe2+ in the chains, coordinated to oxygen atoms from water ligands instead of the nitrogen atoms of the triazole ligand.

In the case of the Sample B the transition is centered closer to room temperature, with a narrower hysteresis of 10 K (T↑_1/2 = 315 K and T↓_1/2 = 305 K)).

5.2. Triazol nanoparticles (SCO complex containing Fe2+) Raman spectroscopy

As aforementioned, the transition from high to low spin is accompanied by the reduction (ford6species a total reduction) of charge in the antibonding e_gorbitals towards t_2g orbitals [1]. Vibrational modes will be affected because of the change in the metal-ligand bond distance (M-L) between the HS and LS state. This will also affect the Fe-N streching mode of the Fe2+, as will be shown below. This can be observed in the vibrational spectrum in the region of ∼ 250 cm−1 (sometimes

at ∼ 500 cm−1) where the stretching frequencies of transition metal compounds

is usually expected [15].

As the temperature is modified, the vibrational bands belonging to the HS and LS species can be easily recognized as relevant vibrational mode signatures de- creasing or increasing in intensity, respectively.

Given the structural complexity of the studied systems (i.e. the presence of mul- tiple organic ligands, triply bridging between two metal centers at a time, forming chains of unknown length) complete assignment of their solid-state Raman spec- tra would be challenging, and sometimes out of the scope of this work. Then, to simplify the problem we focused on the structural changes that occur in the Fe-N₆ coordination sphere during spin transition. Also we identified internal modes of the NH₂-trz and Htrz rings or the modes of the non-coordinating counter anions, however, no special attention is paid to those modes since the thermal hysteresis loop is clearly monitored by the Fe-N stretching modes.

For the characterization we have used the 514.5 nm line of the Ar ion laser. In order to avoid laser heating in our samples, low power has been used during the experiment (7.8 mW) and it has been necessary to wait 1 min between shots. During the temperature sweep we have wait 10 minutes after each temperature step to guaranty the acquisition stability. In Fig. 5.6 we show the evolution of the Raman spectra of the Sample A with the temperature. Roughly speaking, we have observed two peaks (242 and 285 cm−1) that disappears after heating the sample at 383 K. The peaks come back when cooling down the sample, see Fig. 5.7. Raman spectra clearly show the difference between LS and HS states as corroborates previous magnetic measurements.

This feature is a well-known and extensively reported signature of the spin crossover [16]. About the peaks observed they are assigned to the Fe-N stretch- ing and Fe-N-Fe bending vibration modes and therefore related with the state of the Fe2+ ion. We can estimate the T↓_1/2 cooling and T↑_1/2 heating temperatures as the point where the Raman peak reduces its integrated intensity to a half. This is, T↑_1/2 heating would be around 353 K while the T↓_1/2 cooling 333 K. This means a ΔT value of 20 K which is similar to the one extracted from magnetic

Chapter 5. Raman spectroscopy for the study of Spin-Crossover in iron Fe2+/Fe3+complexes

measurements [17].

Figure 5.5: a) [Fe(Htrz)2(trz)](ClO4) Raman spectra while heating from 150 to 363 K (using 514.5 nm excitation). To avoid laser cooling we waited 1 min between shots; b) [Fe(Htrz)2(trz)](ClO4) Raman spectra while cooling the temperature from 353 K to 150 K (using 514.5 nm excitation). To avoid laser heating we waited 1 min between shots.

Even at different temperature and presenting narrower hysteresis the behavior of the Sample B is qualitatively similar. However, Raman spectra suggest that at room temperature this sample is in HS. Then we need to cool the sample down to observe the spin transition. In this sample we have observed the motion of several peaks. Mainly the mode at 195 cm−1 can be associated to angle bending,

δ(N-Fe-N) of the bridge and modes at 214, 280 cm−1can be associated to de Fe-N stretching modes as Smit et al. proposed for similar compounds [18]. We have also identified the presence of other peaks, for example a weak mode at around 460 cm−1. Even the assign of that modes will be difficult with the structural data in literature, but their correlation with the crossover is clear. This is relevant in the sense that we may monitor the crossover without the need of spending time on the peak assignment. This will be an advantage in the characterization of macromolecular compounds, since we may found spin crossover signatures far from the Fe2+ coordination sphere. In a similar way, in the next section we demonstrate the spin state can be monitored via the dynamics of the vibrational modes of the ligand.